1. Field of the Invention
The present invention relates to an optometric apparatus for subjectively examining visual functions of an examinee's eye.
2. Description of Related Art
A vertex distance VD (visual distance) between a back surface of a lens of spectacles which an examinee puts on (namely, a lens wearing reference point) and a corneal vertex of an examinee's eye is generally considered to be 13.75 mm or 12.00 mm in a reference distance. Accordingly, in an optometric apparatus adapted such that various kinds of optical elements are selectively disposed in a test window to present various kinds of optotypes through the test window in order to subjective examination of visual functions such as a refracting power of the examinee's eye and the like, the vertex distance VD between the lens (optical element) back surface and the corneal vertex needs to be checked. Thus, this type of optometric apparatus is provided with a cornea position alignment optical system for allowing check of the vertex distance VD.
FIG. 8 is a schematic sectional view of the cornea position alignment optical system provided in a conventional optometric apparatus. Various kinds of optical elements 104 are selectively disposed in a test window 103 of a lens chamber unit 100, so that the examinee's eye PE is allowed to look at an optotype 110 forward presented through the optical element 104. The lens chamber unit 100 is attached (or is separately provided) with an aligning unit 125 in which a cornea position alignment optical system 120 is mounted. This alignment optical system 120 is constructed of an aligning scale plate 121 placed to be positioned at the side of the eye PE during examinations, a reflection mirror 122, and a reticle plate 123 placed on an optical path in a direction that the mirror 122 reflects light. An examiner takes his position at a distance of 250 mm from the reticle plate 123 and then goes into position so that triangular reticles 124 provided on the reticle plate 123 appears visually aligned with a long scale line S1 provided on the aligning scale plate 121, as shown in FIG. 9. Then, the examiner checks the positional relation of the long line S1 and a plurality of short scale lines S2, formed on both sides of the long line S1, with respect to the corneal vertex. For example, if the corneal vertex appears to coincide with the long line S1, the vertex distance VD is a reference distance of 13.75 mm. The short lines S2 are spaced at intervals corresponding to several vertex distances. According to which short line S2 (or long line S1) the corneal vertex coincides with, the vertex distance VD can be checked (determined). One example of the optometric apparatus provided with the cornea position aligning system of the above type is disclosed in for example Japanese patent unexamined publication No. Hei 6-181888.
In the above mentioned cornea position aligning system, however, an eye OE of the examiner has to be substantially accurately positioned in a place 250 mm apart from the reticle plate 123; otherwise, the checked vertex distance VD would include considerable errors. If the vertex distance VD is incorrect, the results of the refractive power examination or the like would also have errors.